In U.S. Pat. No. 3,566,493 a method was disclosed for forming an aerofoil-shaped rotor blade with two outer walls and a honeycomb core having two different densities of honeycomb.
U.S. Pat. No. 3,349,157 discloses a laminated airfoil structure having spanwise grooves filled with spanwise fibers and the laminate covered with fiberglass.
Many airfoil sections require compound curvatures and have a high fineness ratio, so that conventional metallic design requires expensive machined spars, multiple ribs with a high part count, a large number of fasteners and present a closeout problem. It was found that a composite airfoil member could be obtained that had low weight, high torsional stiffness and excellent bending strength.